This invention relates generally to hollow containers with inert and/or impermeable surfaces and more particularly to hollow plastic containers with inert/impermeable inner surfaces produced by plasma assisted in situ polymerization or surface activation.
Plastic and metal containers have been replacing glass in many applications where easy handling, low weight and non-breakability are needed. Where metal is used, the internal metal surface of the container must often be coated with a polymer to avoid contact of the packaged content with the metal. Therefore, in the case of plastic packages, and also in case of many metal containers, the contact surface with the packaged content typically comprises a polymer.
Polymers to date have had varying degrees of inertness to the packaged content which differ from the inertness of glass. In the case of food packages, surface inertness helps diminish potential desorption of packaging material components into the food, to prevent flavor-absorption, to avoid loss of food constituents through the package walls and to avoid ingress of air or other substances from outside the package. All these characteristics of inertness apply to plastic containers; however, some of these characteristics also apply to metal containers which have been internally coated with a plastic or lacquer system.
Refillable plastic packages add a further dimension to inertness requirements because these packages must withstand washing and refilling. Such containers should not absorb contact materials such as washing agents or foreign materials stored in the container.
Packages for carbonated beverages are also normally pressurized and must withstand considerable mechanical stress in handling. It is therefore difficult for a single material to provide the necessary mechanical stability and the required inertness.
Current plastic packages for carbonated beverages either consist of a single material such as polyethylene terephthalate (PET), or are comprised of multi-layer structures where usually the middle layers provide the barrier properties and the outer layers the mechanical strength properties. Such containers are produced either by co-injection or co-extrusion. To date, plastic containers with an impermeable, dense "glass-like" inner surface have not been able to be produced by conventional methods.
Some polymers, e.g. polyacrylonitrile, are known to have exceptional barrier properties, but can only be used in copolymer form because the homopolymer, which has the most ideal barrier properties, cannot be processed in the form of a container. A further limitation in the practical application of polymers for food or beverage containers is that polymers with high barrier properties, again as exemplified by acrylonitrile, tend to have aggressive/dangerous monomers, which implies that their use is limited for food contact unless full polymerization without detectable extractables can be achieved.
Recycling is yet another dimension with mass produced packages. The reuse of recycled plastic for the same purpose, i.e. to produce new containers by "closed loop" recycling, is an issue which has attracted much attention, and for PET, this has been achieved to date by depolymerizing the recycled material in order to free it of all trace contaminants which might otherwise migrate and come in contact with the container content. An impermeable inner layer, which is the purpose of the invention, would enable recycled material to be reused directly for new containers, i.e. without special treatment such as depolymerization since traces of foreign substances could no longer contact the container's content. This would simplify the "closed loop" recycling process considerably by obviating the need for depolymerization.
Furthermore, recyclability within established recycling systems, both "open loop", i.e. recycling for other uses, or "closed loop", i.e. reuse for same purpose, is necessary for any mass produced package. In "open loop" systems, the normal method is to separate, clean and chop up the plastic into small flakes. The flake is then either melted and used for molding other objects or for fiber production. For this type of recycling, it is important that any contaminant to the main plastic, such as a coating, should effectively be present in negligible quantities and, preferably, be solid and insoluble within the molten plastic so that it can be filtered off prior to sensitive applications, such as fiber production. PET is also recycled in "closed loop" systems by depolymerization and it is important that the coating material should be unchanged by this process, be insoluble in the monomers resulting from the process, and be easily separable from these monomers. An inert, thin organic coating or surface treatment which changes the surface composition of PET, fulfills these criteria.